Field of the Invention
The invention relates to a monitoring device and a method for monitoring a line section using a monitoring device.
Description of the Background Art
Known from EP 0 206 488 A1 and U.S. Pat. No. 6,285,191 B1 are measurement devices for monitoring currents, in particular of batteries in automobiles. In addition, measurement devices of the generic type are also known from U.S. Pat. No. 6,522,123 B2 and US 2005/0151543 A1, U.S. Pat. No. 4,277,749, DE 100 13 345 A1 and U.S. Pat. No. 6,285,191 B1.
According to the disclosure of U.S. Pat. No. 6,285,191 B1, the current of a ground line is monitored in that, in a first alternative, the current is ascertained from the measured voltage drop in the ground cable and the known resistance of the ground line based on Ohm's law. According to a second alternative, the current in the ground cable is ascertained for an unknown resistance of the ground cable by means of a self-calibrating instrumentation amplifier from the voltage drop. Calibration is carried out using a precision resistor and a precision current source. Moreover, the voltage drop in the ground cable is conducted to an input of the instrumentation amplifier by means of an adjustable voltage divider. The voltage divider must likewise be calibrated. The current in the ground cable is calculated from the comparison of the voltage drop at the precision resistor from the output of the instrumentation amplifier and the current source.
EP 0 206 488 A1 discloses measuring the voltage drop in a line through which current passes, in particular in a ground cable, by means of a Kelvin measurement circuit. The amplitude of the current in the line is determined from the voltage drop. To carry out the measurement, in a first step, a constant current source is connected to the line by means of the Kelvin measurement circuit in order to calibrate a measurement instrument with a known current from the current source. Under operating conditions, i.e., when current from a battery flows through the ground cable, or in more general terms the line, the amplitude of the current in the ground cable is ascertained from the measured voltage drop in the ground cable by means of the previously calibrated measurement instrument.